Solar energy is universally available. Sunshine striking the earth on a typical day is estimated to be sufficient to heat millions of homes, potentially conserving substantial fossil fuel and corresponding costs.
Although solar energy is abundant, harnessing it is limited by available technology. Numerous active and passive solar collector systems for residential, commercial and industrial applications have been proposed; however, widespread acceptance of solar collector systems is restricted by high initial, operating and maintenance costs which result in a long payback period. Costly suitable solar collection materials and fabrication techniques, make their purchase uneconomical. Other problems associated with current solar energy collectors are difficult, expensive or impractical retrofitting to existing structures; bulkiness, weight, size and unaesthetic appearance; unsuitability for on-site assembly and installation by building contractors and the do-it-yourself handyperson; requirement of thermal mass, complicated circulation equipment and other accessories for operation; inflexibility for heating selected spaces; requirements for multiple glazings, thick insulating materials, rubber gaskets and other heat loss reduction materials.
Some collectors rely on tortuous flow paths for transporting solar heated fluids or use heat transfer augmentation such as fins, projections, spirals, inserts, cups, cones, disks, baffles, screens, perforated plates and shells and air jets. Such collectors require large fans, blowers or pumps to overcome pressure drops created by inherent flow restrictions.
U.S. Pat. No. 4,403,600, incorporated herein by reference, shows a modular solar heating system capable of being retrofitted to a structure. Air is blown over two surfaces of a heat exchanger (absorber plate) via a divided air space through "slot-like" apertures. Slot resistance creates a pressure drop requiting a large fan and requires cutting a large opening in a building interior wall for a plenum to house the fan. Also, wooden frame construction can be detrimental during off-season periods such as late spring or summer when collector stagnation temperatures are known to approach kindling temperature of wood. Ultra-violet rays can cause degradation of the plastic sheet used in the rear of the collector. Due to inherent thermal inefficiencies, a 4 foot by 8 foot (1.22 m by 2.44 m) collector is used. The collector is large, bulky, difficult to manufacture, unsuitable for site assembly, difficult to install and thereby increases overall system cost.
U.S. Pat. No. 4,300,601, incorporated herein by reference, shows a vacuum valve and monitoring system for evacuated flat plate solar collector units. The device employs a ball switching device to automatically activate and operate a vacuum pump to restore vacuum in a system upon loss of vacuum. This configuration requires a permanently installed vacuum pump near the collector unless the unit is hermetically vacuum sealed.
U.S. Pat. No. 3,990,201, incorporated herein by reference, shows an evacuated dual glazing system of less than 0.25 inch (0.64 cm) spacing between panes. Such small spacing between glazings drastically reduces effective insulating benefits. The patent teaches that 0.25 inch (0.64 cm) spacing between panes is necessary for safety and that mica spacers placed 1 inch (2.54 cm) apart in checkerboard fashion reduce mechanical bending stresses due to atmospheric pressure. "Aluminizing" of interior glazings to minimize heat loss or gain is shown.
U.S. Pat. No. 4,201,195, incorporated herein by reference, utilizes air forced through numerous critically positioned perforations in a "jet plate." The air strikes a corrugated solar heated target absorber plate and transports heat along the channels. Corrugation increases target absorber plate surface area and collection efficiency. This design requires pumping to overcome resistance of the "jet plate." Collector construction does not favor easy manufacture or retrofit with a single stage module.
According to studies such as in "Popular Science," Jun., 1993, article: "Window Technology: Part One . . . No Pane, No Gain" pages 92-98, very little major improvement in glazings or in window technology has occurred for thirty to forty years. Windows remained the same with wood or aluminum frames and glass panes. They let in too much cold in the winter and heat in the summer. To upgrade them, storm windows were added. In the last decade, changes such as coatings that let in light, but not heat; energy saving inert gas filling between panes; suspended plastic film; and heat stopping edge spacers have been used.
Types of insulated windows include:
1. Standard insulated glass: 1/2 inch (1.27 cm) air space; wood vinyl, or metal frame (with thermal break).
2. Low-e coating (emissivity=0.1 to 0.02): 1/2 inch (1.27 cm) space with argon gas fill; quality wood, vinyl or metal frame (with thermal break).
3. Spectrally selective soft coat low-e coating (emissivity=0.04) with low solar transmittance (such as Cardinal Low e2 made by Cardinal Insulated Glass, Minnetonka, Minn.): 1/2 inch (1.27 cm) space with argon gas fill; quality wood, vinyl or metal frame (with thermal break).
4. State-of-the-art superwindow (such as Hurd Insol-8, Pella InsulShield or Weather Shield Super Smart); triple glazing or two suspended films; two low-e coatings; argon or krypton gas fill; quality wood, vinyl or metal frame (with thermal break); very low air leakage.
Glazings perform several functions including transmitting light through them and to isolate the indoor or enclosure environment from outdoors. Most glass is transparent to sunlight. Solar energy moves in and out windows; however, radiative energy transfer occurs in several ways. Infrared radiation, a part of the solar spectrum, is transmitted directly through most windows heating the glass in the process. Heat absorbed is reradiated. Air rises near the warmer glass surface and falls along the cooler surface in the space between the panes by convection. These convection currents can transfer heat from one pane of glass to another. Conduction loss occurs when heat is conducted from the warmer to the cooler side of a window as each molecule excites its neighbor, passing the energy along. Conduction occurs through glass, window frames, and air or inert gas between glass layers. The tighter the window and the better weather stripping or seal, the lower is air leakage that occurs around and through windows and glazings. However, despite attempts at minimizing heat loss from windows, no highly efficient, cost effective window or glazing exists. Ten years ago the standard air space was 1/4 inch (0.64 cm) wide. Simply increasing that width to 1/2 inch (1.27 cm) was reported to improve the R-value (a measure of resistance to heat flow) about 15% because heat has to travel a greater distance. However, when the space exceeds about 5/8 inch (1.59 cm), air can circulate by convection which causes heat loss. This loss can be further reduced by replacing air with a gas that transmits heat less efficiently. Such low conduction gases were first used in welded glass edge insulated glazing units more than twenty years ago by PPG Industries, Pittsburgh, Pa., according to Moe Peterson, Director of Product Development for flat glass at PPG. Libbey Owens Ford used chlorofluorocarbons (CFC's) and carbon dioxide in early products. Argon gas is used by Andersen Corporation of Bayport, Minn. in all of their insulated windows. Although carbon dioxide performs about as well as argon, it is more prone to leak. Sulphur hexafluoride and krypton are other options. Sulphur hexafluoride insulates somewhat better than argon, but the gas has a low viscosity--the molecules slide past each other more easily which means that sulphur hexafluoride begins convecting in spaces smaller than 1/4 inch (0.64 cm). This gas performs better when mixed with argon. Krypton offers the lowest conductivity of any gas in use, and its moderately low viscosity keeps the optimal inter-glazing space around 1/4 inch (0.64 cm). Unfortunately, this gas is also scarce and expensive. Hurd Millwork of Medford, Wis. and Pella Corporation of Pella, Iowa use krypton gas in a few multipane super-high-performance windows. Hurd also uses a mirrored film called Heat Mirror, developed by Southwall Technologies of Palo Alto, Calif., suspended between glass panes to provide an additional air space to slow heat loss. All these window glazing improvements reduced heat loss transmitted by the window thereby increasing the R-value compared to standard glass.
On pages 95 and 96, Tom Potter, at the National Renewable Energy Laboratory in Golden, Colo., says the most energy efficient option of all would be no gas, i.e. a vacuum. Research studies conducted in the mid-to-late 1980's showed very dramatic center-of-glass insulation levels of as high as R-15 with a 1/10 inch (0.25 cm) vacuum space. They report that little further work has been done recently on the technology. Vacuum glazings present many challenges, according to experts Potter and Dave Benson, the greatest being how to maintain the vacuum state. The only successful method demonstrated thus far is one that uses a continuous glass welded edge. Second, spacers must hold the panes of glass away from one another. Tiny glass beads were used in prototype panels, but they could distort the window view. Stresses on edges from differential thermal expansion of inner and outer panes of glass could potentially cause breakage, sending glass shards some distance. Informal lab testing did not show this to be the case, according to Potter. Higher strength tempered glass might prevent such breakage, but this type of glass so far cannot be welded for a vacuum seal, he says.
A need exists for a solar collector system that overcomes the problems and limitations associated with current solar collector systems.